1. Field
The present invention relates to converters, and more particularly to circuits and methods for controlling a converter.
2. Description of the Related Art
Converters are currently widely used in electronic systems for providing regulated power supplies. Typically, converters employ a structure of a switch-mode power supply to achieve higher efficiency, smaller size or lighter weight. There exists a variety of converters, such as a buck converter, a boost converter, and a buck/boost converter.
FIG. 1 illustrates a schematic diagram of a conventional converter circuit 100. In general, the converter circuit 100 converts an input voltage VIN from a power source (e.g., a battery 130) to a regulated output voltage VOUT. For example, the converter circuit 100 may include a buck converter 110 and a controller 120. The buck converter 110 may further include a first switch 102, a second switch 104, an inductor 106 and an output capacitor 108. The first switch 102 is typically coupled to the battery 130 and thus is referred to as a high side switch. The second switch 104 is typically coupled to ground and thus is referred to as a low side switch. In operation, the controller 120 turns the first and second switches 102 and 104 alternately on or off, thereby producing the regulated output voltage VOUT at the output capacitor 108. The controller 120 typically generates pulse width modulated (PWM) signals to control the conduction status of the first and second switches 102 and 104. A duty ratio of the PWM signals determines a voltage level of the output voltage VOUT.
However, the buck converter 110 faces power loss and low efficiency during a shutdown process. For instance, when both the first and second switches 102 and 104 are turned off, the shutdown process starts. During the shutdown process, energy retained in the output capacitor 108 will be discharged through a load (not shown), which is generally coupled in parallel with the output capacitor 108 and in high impedance. As such, power efficiency of the buck converter 110 is degraded and the running time of the battery 130 is shortened. Additionally, because the load is generally in high impedance, there will be a long shutdown process.
FIG. 2 illustrates a schematic diagram of another conventional converter circuit 200. Elements labeled the same in FIG. 1 have similar functions and will not be repetitively described herein for purposes of brevity and clarity. In FIG. 2, the output of the buck converter 110 is coupled to a resistive load 230, which is further coupled to ground via a discharge switch 240. The conduction state of the discharge switch 240 is controlled by a discharge signal 250 asserted by a controller 220. During the shutdown process, the discharge switch 240 is turned on such that the energy retained in the output capacitor 108 can be discharged through the resistive load 230. Though the shutdown process is shortened significantly, the drawbacks regarding power loss and low efficiency still exist.
Another traditional method of shutting a converter down is to ramp down the output voltage VOUT to zero in a controlled manner. For example, the controller operates in a closed loop to cause the output voltage VOUT in relation to an internal reference voltage. When the converter is shut down, the internal reference voltage ramps gradually down to zero, thereby adjusting the duty ratio of the PWM signals. As such, the first and second switches are still turned alternately on or off by the PWM signals during the shutdown process but the output voltage VOUT ramps gradually down to zero due to the dynamically adjusted duty ratio of the PWM signals. In this instance, if there is no load present during the shutdown process, the energy retained in the output capacitor can be brought back to the power source. However, energy transfer from the power source to the converter still happens via the first and second switches. In particular, if there is a load present at the output of the converter during the shutdown process, there will be energy dissipation in the load. The efficiency of the converter circuit is further reduced due to a high switching frequency of the first and second switches during the shutdown process.